(1) Field of the Invention
The present invention relates to a communication device, and more particularly, to a communication device connected with a plurality of couplers on a downlink side thereof, each coupler being connected with a plurality of sub-units.
(2) Description of the Related Art
Currently, with the advent of a real broadband age, FTTH (Fiber to The Home) is being constructed as most prospective means for high-speed access because of its large-capacity/wideband transmission and the capability to provide services of uniform and stable quality.
For such FTTH applications, PON (Passive Optical Network) which comprises a host unit arranged on a central office side and a sub-unit arranged on a subscriber side and connected with a plurality of user terminals is considered an effective network configuration because, with PON, the cost of laying optical fibers can be reduced and also the accommodation efficiency of the central office side can be increased. Also, since the branch point requires neither power supply nor maintenance, PON provides high flexibility in configuring networks.
In the PON architecture, the host unit takes charge of the management of sub-units, such as registration/deletion of sub-units, and each sub-unit notifies the host unit of alarm, link state, link speed, etc. FIG. 19 illustrates a PON system configuration. As shown in FIG. 19, a host unit 101 is connected with couplers 102a, 102m. To the couplers 102a, . . . , 102m are connected a plurality of sub-units 103a, . . . , 103n which in turn are connected with terminals 104a, . . . , 104x. The host unit 101 includes a layer 2 switch 101a for switching a packet path at layer 2 which is the data-link layer in the OSI reference model, buffering sections 101ba, . . . , 101bm, and PON interfaces 101ca, . . . , 101cm. 
Transmission (uplink transmission) of packets from the sub-units 103a, . . . , 103n (terminals 104a, . . . , 104x) to the host unit 101 is started after the sub-units 103a, . . . , 103n are registered and transmission timings and bandwidths therefor are designated by the host unit 101.
When transmitting packets from the host unit 101 to the sub-units 103a, . . . , 103n (downlink transmission), first, the layer 2 switch 101a carries out switching of packets to be output to the PON interfaces 101ca, . . . , 101cm connected with the sub-units (couplers). The PON interfaces 101ca, . . . , 101cm then transmit the packets output from the layer 2 switch 101a to the respective sub-units 103a, . . . , 103n associated therewith. When the layer 2 switch 101a is input with packets with unknown destinations, such packets are output to all PON interfaces 101ca, . . . , 101cm and then to all couplers 102a, . . . , 102m. 
As regards a star network configuration in which a plurality of branch offices (BO's) are connected in star form to a head office (HO) via an ATM (Asynchronous Transfer Mode) network, a star network connection system has been proposed which permits efficient use of the bandwidth of the ATM network in cases where MAC (Media Access Control) frames to be transmitted are broadcast frames or destination MAC address are unlearned addresses (e.g., Unexamined Japanese Patent Publication No. 2003-143174. There has also been proposed an access router for preventing congestion from occurring in a layer 2 section and thereby preventing packets with high priority from being discarded (e.g., Unexamined Japanese Patent Publication No. 2002-271382.
In cases where packets with unknown destinations are input to the layer 2 switch, however, such packets are broadcast and delivered to all sub-units, as mentioned above. Accordingly, although there are actually no terminals that are to receive packets with unknown destinations, the packets are delivered to all sub-units connected to the PON interfaces, wasting the transmission bandwidth and lowering the transmission efficiency.